Display device, electro-optical element driving method and electronic equipment

ABSTRACT

The present invention permits a capacitance value of an electro-optical element such as organic EL element to be arbitrarily set without changing the light extraction efficiency of a pixel. That is, the present invention permits a capacitance value Coled of an organic EL element ( 21 ) to be arbitrarily set by adjusting the light emission area of the organic EL element ( 21 ) without changing the light extraction efficiency of a pixel ( 20 ) in an organic EL display device. The organic EL display device has the pixels ( 20 ) arranged in a matrix form. A light extraction opening ( 56 ) is formed on the surface of the pixel with a light-shielding film (black matrix) ( 57 ). The light extraction opening ( 56 ) has an opening area smaller than the light emission area of the organic EL element ( 21 ).

TECHNICAL FIELD

The present invention relates to a display device having pixels,containing an electro-optical element, arranged in a matrix form, and toan electro-optical element driving method and electronic equipment.

BACKGROUND ART

In recent years, in the field of image display device for displayingimages, organic EL display devices having a number of pixel circuits,containing an electro luminescence element, i.e., organic EL element,which is a so-called current-driven electro-optical element whose lightemission brightness changes in accordance with current value flowingthrough the element, arranged in a matrix form have been developed andcommercialized.

An organic EL element is self-luminous. As a result, an organic ELdisplay device offers several advantages compared with a liquid crystaldisplay device which controls the light intensity from the light source(backlight) by means of pixels, containing liquid crystal cells such ashigh image visibility, no need for backlight and high response speed ofthe element.

An organic EL display device can employ either a simple (passive)-matrixsystem or an active-matrix system driven as with a liquid crystaldisplay device. It should be noted, however, that a simple matrixdisplay device has some problems although simple in construction. Suchproblems include such as difficulty in implementing a largehigh-definition display device.

For this reason, in recent years, the development of active matrixdisplay devices has been going on at a brisk pace. Such display devicescontrol the current flowing through the electro-optical element with anactive element such as insulating gate field effect transistor(typically, thin film transistor; TFT) provided in the same pixelcircuit as the electro-optical element.

In an active matrix organic EL display device, a pixel (pixel circuit)at least includes, in addition to an organic EL element, a drivetransistor adapted to drive the organic EL element, a write transistoradapted to sample an input signal voltage and write the voltage to thepixel, and a holding capacitance connected to the gate of the drivetransistor to hold the input signal voltage written by the writetransistor (refer, for example, to Japanese Patent Laid-Open PublicationNo. 2005-345722).

DISCLOSURE OF INVENTION

In the organic EL display device configured as described above, thedrive transistor is designed to operate in the saturation region.Therefore, the drive transistor functions as a constant current source.As a result, a constant drain-to-source current Ids, given by thefollowing formula (1), is supplied to the organic EL element whose anodeelectrode is connected to the source of the drive transistor:Ids=(1/2)·μ(W/L)Cox(Vgs−Vth)²  (1)

where Vth is the threshold voltage of the drive transistor, μ themobility of the semiconductor thin film making up the drive transistor'schannel, W the channel width, L the channel length, Cox the gatecapacitance per unit area, and Vgs the gate-to-source voltage applied tothe gate relative to the source.

On the other hand, as a gate potential Vg of the drive transistor risesas a result of the writing of an input signal voltage Vsig by the writetransistor through sampling, a source potential Vs of the drivetransistor will rise because of the coupling of the holding capacitanceand the capacitance of the organic EL element. Here, letting thecapacitance value of the holding capacitance be denoted by Ccs, thecapacitance value of the organic EL element by Coled and the incrementof the gate potential Vg of the drive transistor by ΔVg, an incrementΔVs of the source potential Vs of the drive transistor is given by thefollowing formula (2):ΔVs=ΔVg×{Ccs/(Coled+Ccs)}  (2)

As is clear from the formula (2), if the capacitance value Coled of theorganic EL element is sufficiently larger than the capacitance value Ccsof the holding capacitance, the increment ΔVs of the source potential Vsof the drive transistor can be suppressed when the gate potential Vg ofthe drive transistor rises. That is, when sufficiently larger than thecapacitance value Ccs of the holding capacitance, the capacitance valueColed of the organic EL element is advantageous in providing a largegate-to-source potential difference of the drive transistor.

The reason for this is as follows. That is, if a large gate-to-sourcepotential difference of the drive transistor can be provided at the timeof writing of the input signal voltage Vsig by the write transistor, theamplitude of the input signal voltage Vsig written to the pixel can bereduced to the same extent. Hence, power consumption of horizontal drivesystem adapted to supply the input signal voltage Vsig to each pixel viaa signal line can be reduced. As a result, the display device as a wholecan be reduced in power consumption.

In light of the foregoing, it is an object of the present invention toprovide a display device which permits the capacitance value of theelectro-optical element such as organic EL element to be arbitrarily setwithout changing the light extraction efficiency of the pixel, and toprovide an electro-optical element driving method and electronicequipment.

A display device according to the present invention is characterized asfollows. That is, pixels, containing an electro-optical element andlight-shielding film, are arranged in a matrix form. The light-shieldingfilm forms a light extraction opening whose opening area is smaller thanthe light emission area of the electro-optical element. The capacitancevalue of the electro-optical element is set by the light emission areaof the electro-optical element.

An electro-optical element driving method according to the presentinvention is a driving method of an electro-optical element adapted toemit light according to the current. The electro-optical element drivingmethod is characterized as follows. That is, the method drives aplurality of electro-optical elements and brings drive current value ofeach electro-optical element to approximately the same level by varyingthe light emission area between the electro-optical elements.

Electronic equipment according to the present invention is characterizedin having a display device. In the display device, pixels, containing anelectro-optical element and light-shielding film, are arranged in amatrix form. The light-shielding film forms a light extraction openingwhose opening area is smaller than the light emission area of theelectro-optical element. The capacitance value of the electro-opticalelement is set by the light emission area of the electro-opticalelement.

In the display device, electro-optical element driving method andelectronic equipment configured as described above, the capacitancevalue of the electro-optical element is determined by the light-emittingmaterial, the film thickness of the light-emitting layer, and the lightemission area. For this reason, the capacitance value of theelectro-optical element is set to an optimal value by adjusting thelight emission area of the electro-optical element. In this case, theopening area of the light extraction opening formed by thelight-shielding film is smaller than the light emission area of theelectro-optical element. Therefore, even if the light emission area ofthe electro-optical element is changed, the light emission area of thepixel determined by the opening area of the light extraction opening,namely, the light extraction efficiency, will remain unchanged.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a system configuration diagram illustrating the outline of theconfiguration of an active matrix organic EL display device according tothe present invention.

FIG. 2 is a circuit diagram illustrating an example of circuitconfiguration of a pixel (pixel circuit).

FIG. 3 is a characteristic diagram of a drain-to-source voltage Vds vs.drain-to-source current Ids of a drive transistor.

FIG. 4 is a timing waveform diagram for describing the circuit operationof the active matrix organic EL display device according to the presentinvention.

FIG. 5 is a sectional view illustrating an example of sectionalstructure of the pixel.

FIG. 6 is a view illustrating the relationship between the change inlight emission area of an organic EL element and the change in acapacitance value Coled thereof.

FIG. 7 is a perspective view illustrating a television set to which thepresent invention is applied.

FIG. 8 is a perspective view illustrating a digital camera, to which thepresent invention is applied, and (A) is a perspective view as seen fromthe front, and (B) is a perspective view as seen from the rear.

FIG. 9 is a perspective view illustrating a laptop personal computer towhich the present invention is applied.

FIG. 10 is a perspective view illustrating a video camcorder to whichthe present invention is applied.

FIG. 11 is a perspective view illustrating a mobile phone to which thepresent invention is applied, and (A) is a front view of the mobilephone in an open position, (B) is a side view thereof, (C) is a frontview of the mobile phone in a closed discharge, (D) is a left side view,(E) is a right side view, (F) is a top view, and (G) is a bottom view.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described in details withreference to the drawings.

FIG. 1 is a system configuration diagram illustrating the outline of theconfiguration of an active matrix display device according to thepresent invention such as active matrix organic EL display device.

As illustrated in FIG. 1, an organic EL display device 10 according tothe present invention includes a pixel array section 30. The pixel arraysection 30 has pixels (pixel circuits) 20 containing an electroluminescence element, i.e., organic EL element 21 (refer to FIG. 2) as alight emitting element, arranged two-dimensionally in a matrix form. Theorganic EL element 21 is a current-driven electro-optical element whoselight emission brightness changes with change in current value flowingthrough the device.

The pixel array section 30 is typically formed on a transparentinsulating substrate such as glass substrate. The pixel array section 30has scan lines 31-1 to 31-m, drive lines 32-1 to 32-m and first andsecond correction scan lines 33-1 to 33-m and 34-1 to 34-m for each ofthe pixels arranged in m rows by n columns. The pixel array section 30also has signal lines (data lines) 35-1 to 35-n for each pixel column.

Several circuits are disposed around the pixel array section 30. Thesecircuits are a write scan circuit 40 adapted to scan and drive the scanlines 31-1 to 31-m, a drive scan circuit 50 adapted to scan and drivethe drive lines 32-1 to 32-m, first and second correction scan circuits60 and 70 adapted to scan and drive the first and second correction scanlines 33-1 to 33-m and 34-1 to 34-m, and a horizontal drive circuit 80adapted to supply a video signal (data signal, i.e., input signal)appropriate to brightness information to the signal lines 35-1 to 35-n.

To scan and drive the scan lines 31-1 to 31-m, drive lines 32-1 to 32-mand first and second correction scan lines 33-1 to 33-m and 34-1 to34-m, the write scan circuit 40, drive scan circuit 50 and first andsecond correction scan circuits 60 and 70 output, as appropriate, writesignals WS1 to WSm, drive signals DS1 to DSm and first and secondcorrection scan signals AZ11 to AZ1 m and AZ21 to AZ2 m.

Each of the pixels 20 of the pixel array section 30 can be formed withan amorphous silicon TFT (thin film transistor) or low-temperaturepolysilicon TFT. Here, a case will be described as an example where thepixels 20 are formed with low-temperature polysilicon TFTs. In the casewhere low-temperature polysilicon TFTs are used, the write scan circuit40, drive scan circuit 50, first and second correction scan circuits 60and 70 and horizontal drive circuit 80 can also be formed integrally ona panel (substrate) on which pixel array section 30 is formed.

(Pixel Circuit)

FIG. 2 is a circuit diagram illustrating an example of circuitconfiguration of the pixel (pixel circuit) 20. As illustrated in FIG. 2,the pixel 20 includes, in addition to the current-driven electro-opticalelement, i.e., organic EL element 21, a drive transistor 22, write(sampling) transistor 23, switching transistors 24 to 26 and holdingcapacitance 27, as its circuit components.

In the pixel 20 configured as described above, N-channel TFTs are usedas the drive transistor 22, write transistor 23 and switchingtransistors 25 and 26. A P-channel TFT is used as the switchingtransistor 24. It should be noted, however, that the combination ofconductivity types of the drive transistor 22, write transistor 23 andswitching transistors 24 to 26 given here is merely an example, and thepresent invention is not limited to this combination.

The organic EL element 21 has its cathode electrode connected to asource potential VSS (ground potential GND in this case). The drivetransistor 22 is adapted to current-drive the organic EL element 21. Thedrive transistor 22 has its source connected to the anode electrode ofthe organic EL element 21, thus forming a source-follower circuit. Thatis, the source potential Vs of the drive transistor 22 is determined bythe operating point between the drive transistor 22 and organic ELelement 21 as illustrated in FIG. 3. The source potential Vs has adifferent voltage value depending on the gate potential Vg.

The write transistor 23 has its source connected to the signal line 35(35-1 to 35-n), its drain connected to the gate of the drive transistor22, and its gate connected to the scan line 31 (31-1 to 31-m). Theswitching transistor 24 has its source connected to a second sourcepotential VDD (positive source potential in this case), its drainconnected to the drain of the drive transistor 22, and its gateconnected to the drive line 32 (32-1 to 32-m). The switching transistor25 has its drain connected to a third source potential Vini1, its sourceconnected to the drain of the write transistor 23 (gate of the drivetransistor 22), and its gate connected to the first correction scan line33 (33-1 to 33-m).

The switching transistor 26 has its drain connected to a connection nodeN11 between the source of the drive transistor 22 and the anodeelectrode of the organic EL element 21, its source connected to a fourthsource potential Vini2 (negative source potential in this case), and itsgate connected to the second correction scan line 34 (34-1 to 34-m). Theholding capacitance 27 has one end connected to a connection node N12between the gate of the drive transistor 22 and the drain of the writetransistor 23. The holding capacitance 27 has the other end connected tothe connection node N11 between the source of the drive transistor 22and the anode electrode of the organic EL element 21.

In the pixel 20 whose components are connected according to the aboveconnection relationship, each of the components serves the followingfunction. That is, the write transistor 23 conducts to sample the inputsignal voltage Vsig supplied via the signal line 35 and write the inputsignal voltage Vsig to the pixel 20. The written input signal voltageVsig is held by the holding capacitance 27. The switching transistor 24conducts to supply a current to the drive transistor 22 from the sourcepotential VDD.

When the switching transistor 24 is conducting, the drive transistor 22supplies a current appropriate to the input signal voltage Vsig held bythe holding capacitance 27 to the organic EL element 21, thus drivingthe same organic EL element 21 (current driving). The switchingtransistors 25 and 26 conduct as appropriate to detect the thresholdvoltage Vth of the drive transistor 22 ahead of the current driving ofthe organic EL element 21 and hold the detected threshold voltage Vth inthe holding capacitance 27 so as to cancel the impact of the currentdriving in advance. The holding capacitance 27 holds the gate-to-sourcepotential difference of the drive transistor 22 over the display period.

As a condition to guarantee the proper operation of the pixel 20, thefourth source potential Vini2 is set lower than the potential obtainedby subtracting the threshold voltage Vth of the drive transistor 22 fromthe third source potential Vini1. That is, the level relationship,Vini2<Vini1−Vth, holds. Further, the level obtained by adding athreshold voltage Vthel of the organic EL element 21 to a cathodepotential Vcat (ground potential GND in this case) of the organic ELelement 21 is set higher than the level obtained by subtracting thethreshold voltage Vth of the drive transistor 22 from the third sourcepotential Vini1. That is, the level relationship, Vcat+Vthel>Vini1−Vth(>Vini2), holds.

[Description of the Circuit Operation]

A description will be given next of the circuit operation of the activematrix organic EL display device 10 having the pixels 20 configured asdescribed above arranged two-dimensionally in a matrix form withreference to the timing waveform diagram in FIG. 4.

FIG. 4 illustrates the timing relationship between the write signal WS(WS1 to WSm) given to the pixel 20 by the write scan circuit 40, thedrive signal DS (DS1 to DSm) given to the pixel 20 by the drive scancircuit 50, and the first and second correction scan signals AZ1 (AZ11to AZ1 m) and AZ2 (AZ21 to AZ2 m) given to the pixel 20 by the first andsecond correction scan circuits 60 and 70, and the changes of the gatepotential Vg and source potential Vs of the drive transistor 22 when thepixels 20 on a column are driven.

Here, the write transistor 23 and switching transistors 25 and 26 areN-channel transistors. Therefore, the write signal WS and first andsecond correction scan signals AZ1 and AZ2 are in an active state athigh level (source potential VDD in this example; hereinafter written as“H” level) and in an inactive state at low level (source potential VSS(GND) in this example; hereinafter written as “L” level). Further, theswitching transistor 24 is a P-channel transistor. Therefore, the drivesignal DS is in an active state at “L” level and in an inactive state at“H” level.

At time t1, the drive signal DS changes from “L” level to “H” level,bringing the switching transistor 24 out of conduction. At time t2, thesecond correction scan signal AZ2 changes from “L” level to “H” level,bringing the switching transistor 26 into conduction. As a result, thesource potential Vini2 is applied to the source of the drive transistor22 via the switching transistor 26.

At this time, the level relationship, Vini2<Vcat+Vthel, holds asmentioned earlier. Therefore, the organic EL element 21 is in areverse-biased state. As a result, no current flows through the organicEL element 21, causing it not to emit light.

Next, at time t3, the first correction scan signal AZ1 changes from “L”level to “H” level, bringing the switching transistor 25 intoconduction. Therefore, the source potential Vini1 is applied to the gateof the drive transistor 22 via the switching transistor 25. At thistime, the gate-to-source voltage Vgs of the drive transistor 22 takes onthe value of Vini1−Vini2. Here, the level relationship, Vini1−Vini2>Vth,is satisfied.

(Vth Correction Period)

Next, at time t4, the second correction scan signal AZ2 changes from “H”level to “L” level, bringing the switching transistor 26 out ofconduction. Then, at time t5, the drive signal DS changes from “H” levelto “L” level, bringing the switching transistor 24 into conduction. As aresult, a current appropriate to the gate-to-source potential differenceVgs of the drive transistor 22 flows through the drive transistor 22.

At this time, the cathode potential Vcat (source potential VSS) of theorganic EL element 21 is higher than the source potential Vs of thedrive transistor 22. Therefore, the organic EL element 21 is in areverse-biased state. As a result, the current from the drive transistor22 flows in the following order, i.e., the node N11, holding capacitance27, node N12, switching transistor 25, and source potential Vini1.Therefore, a charge appropriate to the current is stored in the holdingcapacitance 27. On the other hand, as the holding capacitance 27 ischarged, the source potential Vs of the drive transistor 22 will risegradually from the source potential Vini2 over time.

Then, when, after elapse of a given time, the gate-to-source(N11-to-N12) potential difference Vgs of the drive transistor 22 becomesequal to the threshold voltage Vth of the same drive transistor 22, thesame drive transistor 22 will go into cutoff. Therefore, a current stopsflowing through the drive transistor 22. As a result, the gate-to-source(N11-to-N12) potential difference Vgs of the drive transistor 22, i.e.,the threshold voltage Vth, is held by the holding capacitance 27 as athreshold correction potential.

Then, at time t6, the drive signal DS changes from “L” level to “H”level, bringing the switching transistor 24 out of conduction. Thisperiod from time t5 to time t6 is a period of time during which thethreshold voltage Vth of the drive transistor 22 is detected and held bythe holding capacitance 27. Here, this given period t5 to t6 will bereferred to as the Vth correction period for the sake of convenience.Then, at time t7, the first correction scan signal AZ1 changes from “H”level to “L” level, bringing the switching transistor 25 out ofconduction.

(Write Period)

Next, at time t8, the write signal WS changes from “L” level to “H”level, causing the write transistor 23 to sample the input signalvoltage Vsig and write this signal to the pixel. As a result, the gatepotential Vg of the drive transistor 22 becomes equal to the inputsignal voltage Vsig. The input signal voltage Vsig is held by theholding capacitance 27.

At this time, the source potential Vs of the drive transistor 22 risesdue to the capacitive coupling between the holding capacitance 27 andorganic EL element 21 relative to the amplitude of the gate potential Vgof the drive transistor 22 at the time of sampling by the writetransistor 23. The increment ΔVs of the source potential Vs of the drivetransistor 22 is expressed by the formula (2) mentioned earlier.

The input signal voltage Vsig written by the write transistor 23 is heldby the holding capacitance 27 so that the input signal voltage Vsig isadded to the threshold voltage Vth held by the holding capacitance 27.At this time, the voltage held by the holding capacitance 27 is equal toVsig−Vini1+Vth. Here, for easier understanding, we assume that Vini1=0V. Then, the gate-to-source voltage Vgs is equal to Vsig+Vth.

As described above, the variation of the threshold voltage Vth of thedrive transistor 22 between pixels and the change of the thresholdvoltage Vth over time can be corrected by holding the threshold voltageVth in the holding capacitance 27 in advance. That is, when the drivetransistor 22 is driven by the input signal voltage Vsig, the thresholdvoltage Vth of the drive transistor 22 and the threshold voltage Vthheld by the holding capacitance 27 cancel each other. In other words,the threshold voltage Vth is corrected.

This correction operation of the threshold voltage Vth permitscancellation of the impact of the threshold voltage Vth on the drivingof the organic EL element 21 by the drive transistor 22 even if there isa variation of the threshold voltage Vth between pixels or a change ofthe threshold voltage Vth over time. As a result, the light emissionbrightness of the organic EL element 21 can be maintained constantwithout being affected by the variation of the threshold voltage Vth orthe change thereof over time.

(Mobility Correction Period)

Then, at time t9, the drive signal DS changes from “H” level to “L”level with the write transistor 23 remaining in conduction, bringing theswitching transistor 24 into conduction. As a result, the supply of acurrent from the source potential VDD to the drive transistor 22 begins.It should be noted that this period from time t8 to time t9 is onehorizontal interval (1H). Here, the organic EL element 21 is put into areverse-biased state by setting Vini1−Vth<Vthel.

When the organic EL element 21 is put into a reverse-biased state, theorganic EL element 21 exhibits a simple capacitive characteristic ratherthan diode characteristic. Therefore, the drain-to-source current Idsflowing through the drive transistor 22 is written to a combinedcapacitance C (=Ccs+Coled) of the capacitance value Ccs of the holdingcapacitance 27 and the capacitance value Coled of the organic EL element21. This writing causes the source potential Vs of the drive transistor22 to rise.

The increment ΔVs of the source potential Vs of the drive transistor 22acts so that it is subtracted from the gate-to-source potentialdifference Vgs of the drive transistor 22 held by the holdingcapacitance 27, in other words, in such a manner as to discharge thecharge stored in the holding capacitance 27. This means that a negativefeedback is applied. That is, the increment ΔVs of the source potentialVs of the drive transistor 22 is a feedback amount of the negativefeedback. At this time, the gate-to-source potential difference Vgs ofthe drive transistor 22 is Vsig−ΔVs+Vth.

As described above, if the current flowing through the drive transistor22 (drain-to-source current Ids) is negatively fed back to the gateinput (gate-to-source potential difference) of the drive transistor 22,the dependence of the drain-to-source current Ids of the drivetransistor 22 on the mobility μ in each of the pixels 20 can becancelled. That is, the variation of the mobility μ of the drivetransistor 22 can be corrected.

A period T (t9 to t10) during which the active period of the writesignal WS (“H” level period) and the active period of the drive signalDS (“L” level period) overlap, namely, the overlapping period duringwhich the write transistor 23 and switching transistor 24 are bothconducting, is referred to as a mobility correction period.

Here, a drive transistor with the high mobility μ and another drivetransistor with the low mobility μ are considered. The source potentialVs of the drive transistor with the high mobility μ rises sharply ascompared to that of the drive transistor with the low mobility μ in thismobility correction period T. Further, the larger the source potentialVs is, the smaller the gate-to-source potential difference of the drivetransistor 22 becomes. As a result, a current is less likely to flow.

That is, it is possible to cause the same drain-to-source current Ids toflow through the drive transistors 22 with the different mobilities μ byadjusting the mobility correction period T. The gate-to-source potentialdifference Vgs of the drive transistor 22 determined in the mobilitycorrection period T is retained by the holding capacitance 27. Thecurrent (drain-to-source current Ids) appropriate to the gate-to-sourcepotential difference Vgs flows from the drive transistor 22 to theorganic EL element 21. This allows the organic EL element 21 to emitlight.

(Light Emission Period)

At time t10, the write signal WS falls to “L” level, bringing the writetransistor 23 out of conduction. As a result, the mobility correctionperiod T ends, and a light emission period begins. In the light emissionperiod, the source potential Vs of the drive transistor 22 rises to thedriving voltage of the organic EL element 21. As a result of the rise ofthe source potential Vs, the gate of the drive transistor 22 isdisconnected from the signal line 35 (35-1 to 35-n) and left in afloating state. Therefore, the gate potential Vg will also rise via theholding capacitance 27.

At this time, letting the parasitic capacitance of the gate of the drivetransistor 22 be denoted by Cg, the increment ΔVg of the gate potentialVg is expressed by the following formula (3):ΔVg=ΔVs×{Ccs/(Ccs+Cg)}  (3)

During this period, the gate-to-source potential difference Vgs held inthe holding capacitance 27 maintains the value of Vsig−ΔVs+Vth.

Then, as the source potential Vs of the drive transistor 22 rises, thereverse bias is removed from the organic EL element 21. Therefore, theconstant drain-to-source current Ids given by the aforementioned formula(1) flows from the drive transistor 22 to the organic EL element 21,causing the organic EL element 21 to actually start emitting light.

The relationship between the drain-to-source current Ids andgate-to-source potential difference Vgs at this time is given by thefollowing formula (4) by substituting Vsig−ΔVs+Vth into Vgs in theformula (1).

$\begin{matrix}\begin{matrix}{{Ids} = {k\;{\mu\left( {{V\;{gs}} - {V\;{th}}} \right)}^{2}}} \\{= {k\;{\mu\left( {{V\;{sig}} - {\Delta\; V}} \right)}^{2}}}\end{matrix} & (4)\end{matrix}$

In the above formula (4), k=(1/2)(W/L)Cox.

As is clear from the formula (4), the term of the threshold voltage Vthof the drive transistor 22 is cancelled. The drain-to-source current Idssupplied from the drive transistor 22 to the organic EL element 21 isindependent of the threshold voltage Vth of the drive transistor 22.Basically, the drain-to-source current Ids is determined by the inputsignal voltage Vsig. In other words, the organic EL element 21 emitslight at the brightness appropriate to the input signal voltage Vsigwithout being affected by the variation of the threshold voltage Vth ofthe drive transistor 22 or the change thereof over time.

Further, as is clear from the formula (4), the input signal voltage Vsigis corrected with the feedback amount ΔVs by negatively feeding back thedrain-to-source current Ids to the gate input of the drive transistor22. The feedback amount ΔVs acts to cancel the effect of the mobility μin the coefficient part of the formula (4). Therefore, thedrain-to-source current Ids is substantially dependent only on the inputsignal voltage Vsig. That is, the organic EL element 21 emits light atthe brightness appropriate to the input signal voltage Vsig withoutbeing affected by the variation of the threshold voltage Vth of thedrive transistor 22 or mobility μ of the drive transistor 22 or thechange thereof over time. This provides uniform image quality free frombanding or uneven brightness.

Here, in the active matrix display device having the pixels 20,containing a current-driven electro-optical element, i.e., the organicEL element 21, arranged in a matrix form, if the light emission time ofthe organic EL element 21 is long, the I-V characteristic of the organicelement 21 will change. For this reason, the connection node N11 betweenthe anode electrode of the organic EL element 21 and the source of thedrive transistor 22 will also change in potential.

In contrast, in the active matrix organic EL display device 10configured as described above, the gate-to-source potential differenceVgs of the drive transistor 22 is maintained constant. For this reason,the current flowing through the organic EL element remains unchanged.Therefore, the constant drain-to-source current Ids will continue toflow through the organic EL element 21 even if the I-V characteristic ofthe organic EL element 21 deteriorates. As a result, the light emissionbrightness of the organic EL element 21 will remain unchanged(compensation function for a characteristic change of the organic ELelement 21).

Further, the threshold voltage Vth of the drive transistor 22 is held bythe holding capacitance 27 before the writing of the input signalvoltage Vsig. As a result, the threshold voltage Vth of the drivetransistor 22 can be cancelled (corrected) so that the constantdrain-to-source current Ids flows through the organic EL element 21without being affected by the variation of the threshold voltage Vth orthe change thereof over time. This provides a high quality display image(compensation function for the variation of Vth of the drive transistor22).

Still further, in the mobility correction period t9 to t10, thedrain-to-source current Ids is negatively fed back to the gate input ofthe drive transistor 22 so that the input signal voltage Vsig iscorrected with the feedback amount ΔVs. As a result, the dependence ofthe drain-to-source current Ids of the drive transistor 22 on themobility μ is cancelled, thus allowing the drain-to-source current Ids,which is dependent only on the input signal voltage Vsig, to flowthrough the organic EL element 21. This ensures uniform display imagequality free from banding or uneven brightness caused by the variationof the mobility μ of the drive transistor 22 or the change thereof overtime (compensation function for the mobility μ of the drive transistor22).

Incidentally, if the capacitance value Coled of the organic EL element21 is sufficiently larger than the capacitance value Ccs of the holdingcapacitance 27, the increment ΔVs of the source potential Vs of thedrive transistor can be suppressed when the gate potential Vg of thedrive transistor rises as described earlier. Therefore, whensufficiently larger than the capacitance value Ccs, the capacitancevalue Coled is advantageous in providing the large gate-to-sourcepotential difference Vgs of the drive transistor.

For this reason, the present invention is characterized in that itpermits the capacitance value Coled of the organic EL element 21 to bearbitrarily set without changing the light extraction efficiency of thepixel 20 so that the capacitance value Coled of the organic EL element21 can be set sufficiently larger than the capacitance value Ccs of theholding capacitance 27.

(Pixel Structure)

FIG. 5 is a sectional view illustrating an example of sectionalstructure of the pixel 20. As illustrated in FIG. 5, the pixel 20includes a substrate 51 on which the drive transistor 22, writetransistor 23, switching transistors 24 to 26 and other components areformed. The pixel 20 further includes an insulating film 52 formed onthe substrate 51 and is configured to have the organic EL element 21disposed in a concave portion 52A of the insulating film 52.

The organic EL, element 21 includes a first electrode (e.g., anodeelectrode) 53 made up of a metal or other substance formed on the bottomportion of the concave portion 52A of the insulating film 52. Theorganic EL element 21 further includes an organic layer 54 formed on thefirst electrode 53 and a second electrode (e.g., cathode electrode) 55formed commonly for all the pixels on the organic layer 54 and made up,for example, of a transparent conductive film.

In the organic EL element 21, the organic layer 54 is formed by stackinga hole transporting layer, light-emitting layer, electron transportinglayer and electron injection layer successively in this order on thefirst electrode 53. Then, a current flows from the drive transistor 22shown in FIG. 2 to the organic layer 54 via the first electrode (anodeelectrode) 53. This causes electrons and holes to recombine in thelight-emitting layer of the organic layer 54, thus causing light to beemitted.

On the top surface of the organic EL element 21, i.e., the top surfaceof the second electrode (transparent electrode) 55 is formed with alight-shielding film 57 which is referred to as so-called “blackmatrix.” The light-shielding film 57, which is patterned on apixel-by-pixel basis, forms a light extraction opening 56 whose openingarea is smaller than the light emission area of the organic EL element21, i.e., the surface area of the organic layer 54. The light-shieldingfilm 57 acts to suppress the optical interference between the adjacentpixels, thus providing improved contrast ratio.

That is, when the organic EL element 21 emits light, the area from whichlight can be extracted is the opening area of the light extractionopening 56 where the light-shielding film 57 is not disposed. That is,light cannot be extracted from the organic EL element 21 located wherethe light-shielding film 57 is disposed, in other words, outside thelight extraction opening 56 even if the organic EL element 21 emitslight. That is, the opening area of the light extraction opening 56 isthe light emission area of the pixel 20.

In the pixel 20 configured as described above, the capacitance valueColed of the organic EL element 21 is proportional to the light emissionarea of the organic EL element 21 (surface area of the organic layer54). Therefore, the capacitance value Coled of the organic EL element 21can be increased by increasing the light emission area of the organic ELelement 21. More specifically, if the light emission area of the organicEL element 21 is increased from the condition in FIG. 6A to that in FIG.6B, the capacitance value Coled of the organic EL element 21 willincrease by as much as the light emission area is increased (higher ELcapacitance).

Even if the light emission area of the organic EL element 21 isincreased, the light extraction efficiency of the pixel 20 will remainthe same as before the light emission area of the organic EL element 21is increased. The reason for this is as follows. That is, when the lightemission area of the organic EL element 21 is increased, the area of theorganic layer 54 will spread out under the light-shielding film 57. Thelight emitted by the spread portion is shielded by the light-shieldingfilm 57. Therefore, the light extraction efficiency of the pixel 20 isdetermined by the opening area of the light extraction opening 56,irrespective of the light emission area of the organic EL element 21(surface area of the organic layer 54).

As described above, the organic EL display device has the pixels 20arranged in a matrix form. The pixel 20 has the light extraction opening56 formed therein whose opening area is smaller than the light emissionarea of the organic EL element 21. The light extraction opening 56 isformed by the light-shielding film (black matrix) 57 on the pixelsurface. In this organic EL display device, the capacitance value Coledof the organic EL element 21 can be arbitrarily set by adjusting thelight emission area of the organic EL element 21. This permits thecapacitance value Coled of the organic EL element 21 to be setsufficiently larger than the capacitance value Ccs of the holdingcapacitance 27 without changing the light extraction efficiency of thepixel 20.

If the capacitance value Coled of the organic EL element is sufficientlylarger than the capacitance value Ccs of the holding capacitance, theincrement ΔVs of the source potential Vs can be suppressed when the gatepotential Vg of the drive transistor 22 rises as is clear from theformula (2). This provides the large gate-to-source potential differenceVgs of the drive transistor 22.

As described above, if the large gate-to-source potential difference Vgsof the drive transistor 22 can be provided at the time of writing of theinput signal voltage Vsig by the write transistor 22, the amplitude ofthe input signal voltage Vsig written to the pixel 20 can be reduced tothe same extent. Hence, the horizontal drive circuit 80, adapted tosupply the input signal voltage Vsig to each pixel 20 on the rowselected by the write scan circuit 40 via the signal line 35 (35-1 to35-n), can be reduced in power consumption. As a result, the displaydevice as a whole can be reduced in power consumption.

On the other hand, if, in the color organic EL display device, thepixels 20 are arranged so that at least two colors, and for examplethree colors, namely, R (red), G (greed) and B (blue), are grouped asone unit, the organic EL elements 21 adapted to emit the respectivecolors have the different capacitance values Coled because they are madeof different materials and have different film thicknesses.

In the organic EL element 21 whose structure is shown in FIG. 5, theemission color is determined, for example, by the material used to formthe light-emitting layer of the organic layer 54 and a thickness t ofthe organic layer 54. In other words, the organic EL elements adapted toemit each color of R, G and B differ in the material of the organic ELelement or film thickness t from one another. The difference in thematerial of the organic EL element or film thickness t changes thecapacitance Coled of the organic EL element. That is, the difference inthe material or film thickness t between the organic EL elements adaptedto emit the respective colors leads to a difference in the capacitanceColed therebetween.

As an example, the relationship in magnitude of the film thickness t ofthe organic EL element, R>G>B, holds because of the relationship inmagnitude of wavelength (R>G>B) between the three colors of R, G and B.Therefore, the relationship in magnitude of the capacitance Coled of theorganic EL element, B>G>R, holds, with B having the largest capacitanceColed, which is the opposite of the film thickness. The ratio ofcapacitance is, for example, R:G:B=1:1.2:1.5.

For this reason, the light emission area is adjusted for each of theorganic EL elements adapted to emit each color of R, G and B. Forexample, the light emission areas of G and R are increased successivelyin this order relative to the light emission area of B so that thecapacitance Coled of the organic EL element is the same for all thecolors. As a result, when the gate potential Vg of the drive transistor22 rises as a result of the writing of the signal voltage Vsig by thewrite transistor 23 through sampling, the increment ΔVs of the sourcepotential Vs of the drive transistor 22 resulting from the coupling ofthe holding capacitance 27 and the capacitance of the organic ELelements 21 will be the same between each pixel of R, G and B.

Further, if the capacitance Coled of the organic EL element 21 isdifferent depending on the emission color, the increment ΔVs of thesource potential Vs of the drive transistor 22 will be different betweenR, G and B even when the increment ΔVg of the gate potential Vg of thedrive transistor 22 is the same for R, G and B, as is clear from theformula (2) described earlier. As a result, even if the input signalvoltage Vsig of the same level (voltage value) is fed to each R, G and Bpixel, the drive voltages of each R, G and B organic EL element will notreach the voltage value appropriate to the signal voltage Vsig, thusresulting in an improper white balance.

The term “improper white balance” means that even if the input signalvoltage Vsig adapted to display white is fed to each R, G and B pixel,the display colors of each R, G and B pixel will not combine intocompletely white. An improper white balance makes it impossible toproduce images with natural-looking color.

Also in such a case, the light emission area is varied between theorganic EL elements adapted to emit each color of R, G and B (betweenthe pixels emitting the different colors). By doing so, the changebetween the gate-to-source potential difference Vgs when the inputsignal is written and the gate-to-source potential difference Vgs whenthe organic EL element 21 emits light is adjusted. As a result, thedrive voltages of the organic EL elements 21 of the respective colorswill be voltage values appropriate to the input signal voltage Vsig inaccordance with the input of the input signal voltage Vsig of the samelevel. This ensures that the drive current values of the organic ELelements 21 of the respective colors will be approximately the same,thus allowing maintaining a proper white balance. As a result, imageswith more natural-looking color can be produced.

Here, red, green and blue were used as a plurality of the three basiccolors as a unit for image display. However, the present invention isnot limited to the combination of these three colors, but other colorsuch as white may be added to the three colors to produce a four-colorcombination. Alternatively, other colors may be combined together.

It should be noted that the pixel circuit (pixel) of the organic ELdisplay device to which the present invention is applicable is notlimited to the example of the pixel circuit shown in FIG. 2. Instead,the present invention is applicable to all circuits in which the sourcepotential Vs of the drive transistor 22 rises because of the coupling ofthe holding capacitance 22 and the capacitance of the organic EL element21 at the time of writing of the signal to the gate of the drivetransistor 22.

Further, although the above embodiment has been described taking, as anexample, a case in which the present invention is applied to the organicEL display device using the organic EL element 21 as an electro-opticalelement of the pixel 20, the present invention is not limited to thisapplication example. Instead, the present invention is applicable to alldisplay devices in general using current-driven electro-optical elements(luminescence elements) whose light emission brightness changes withchange in a current value flowing through the device.

APPLICATION EXAMPLES

The display device according to the present invention described above isapplicable as a display device of electronic equipment across all fieldsincluding those shown in FIGS. 7 to 11, namely, a digital camera, laptoppersonal computer, mobile terminal device such as mobile phone and videocamcorder. These pieces of equipment are designed to display an image orvideo of a video signal fed to or generated inside the electronicequipment. Examples of electronic equipment to which the presentinvention is applied will be described below.

It should be noted that the display device according to the presentinvention includes that in a modular form having a sealed configuration.For example, such a display device corresponds to a display moduleformed by attaching an opposed section made, for example, of transparentglass to the pixel array section 30. The aforementioned light-shieldingfilm may be provided on the transparent opposed section, in addition tofilms such as color filter and protective film. It should be noted thata circuit section, FPC (flexible printed circuit) or other circuitry,adapted to allow exchange of signals or other information betweenexternal equipment and the pixel array section, may be provided on thedisplay module.

FIG. 7 is a perspective view illustrating a television set to which thepresent invention is applied. The television set according to thepresent application example includes a video display screen section 101made up, for example, of a front panel 102, filter glass 103 and otherparts. The television set is manufactured by using the display deviceaccording to the present invention as the video display screen section101.

FIG. 8 is a perspective view illustrating a digital camera to which thepresent invention is applied. (A) is a perspective view of the digitalcamera as seen from the front side, and (B) is a perspective viewthereof as seen from the rear side. The digital camera according to thepresent application example includes a light-emitting section 111 forflash, display section 112, menu switch 113, shutter button 114 andother parts. The digital camera is manufactured by using the displaydevice according to the present invention as the display section 112.

FIG. 9 is a perspective view illustrating a laptop personal computer towhich the present invention is applied. The laptop personal computeraccording to the present application example includes, in a main body121, a keyboard 122 adapted to be manipulated for entry of text or otherinformation, a display section 123 adapted to display an image, andother parts. The laptop personal computer is manufactured by using thedisplay device according to the present invention as the display section123.

FIG. 10 is a perspective view illustrating a video camcorder to whichthe present invention is applied. The video camcorder according to thepresent application example includes a main body section 131, lens 132provided on the front-facing side surface to image the subject, imagingstart/stop switch 133, display section 134 and other parts. The videocamcorder is manufactured by using the display device according to thepresent invention as the display section 134.

FIG. 11 is a perspective view illustrating a mobile terminal device suchas mobile phone to which the present invention is applied. (A) is afront view of the mobile phone in an open position. (B) is a side viewthereof. (C) is a front view of the mobile phone in a closed discharge.(D) is a left side view. (E) is a right side view. (F) is a top view.(G) is a bottom view. The mobile phone according to the presentapplication example includes an upper enclosure 141, lower enclosure142, connecting section (hinge section in this example) 143, display144, sub-display 145, picture light 146, camera 147 and other parts. Themobile phone is manufactured by using the display device according tothe present invention as the display 144 and sub-display 145.

The present invention permits the capacitance value of theelectro-optical element to be arbitrarily set without changing the lightextraction efficiency of the pixel by adjusting the light emission areaof the electro-optical element.

1. A display device having pixels arranged in a matrix form, each of thepixels comprising: an electro-optical element; a drive transistoradapted to drive the electro-optical element; a holding capacitanceadapted to hold an input signal voltage supplied to the drivetransistor; and a light-shielding film disposed adjacent to a lightextraction area, the light extraction area being smaller than a lightemission area of the electro-optical element, wherein a capacitancevalue of the electro-optical element formed by the light emission areaof the electro-optical element is set larger than a capacitance value ofthe holding capacitance, a unit includes at least two pixels that havedifferent light emission areas associated with different emissioncolors, and the capacitance value of the electro-optical element is thesame for the pixels in the unit.
 2. A display device having pixels, eachof the pixels comprising at least: an electro-optical element; and alight-shielding film disposed adjacent to a light extraction area, thelight extraction area being smaller than a light emission area of theelectro-optical element, wherein the pixels are arranged in a matrixform so that at least two electro-optical elements having differentlight emission areas are grouped as a unit, and capacitance values ofthe electro-optical elements of the pixels are set equal to each otherby the light emission areas.
 3. An electro-optical element drivingmethod for driving an electro-optical element adapted to emit lightaccording to a current, the electro-optical element driving methodcomprising: holding an input voltage to be supplied to a gate of a drivetransistor in a holding capacitance connected between the gate and aconnection node on a source side of the drive transistor; supplying acurrent appropriate to the input voltage held by the holding capacitancefrom the drive transistor to the electro-optical element which isconnected to the connection node on the source side and has thecapacitance value larger than that of the holding capacitance; causingthe electro-optical element to emit light via a light extraction areawhere the light extraction area is smaller than the light emission areaof the electro-optical element; composing a unit including at least twopixels that have different light emission areas associated withdifferent emission colors; and setting the capacitance value of theelectro-optical element the same for the pixels in the unit.
 4. Anelectro-optical element driving method for driving a plurality ofelectro-optical elements adapted to emit different colors as a unit, theelectro-optical element driving method comprising: supplying a drivecurrent of approximately identical value to the electro-optical elementsadapted to emit the respective colors whose light emission areas aredifferent from each other; causing the electro-optical elements adaptedto emit the respective colors to emit light via light extraction areaswhere the light extraction areas are smaller than the light emissionareas of the electro-optical elements; composing a unit including atleast two pixels that have different light emission areas associatedwith different emission colors; and setting the capacitance value of theelectro-optical element the same for the pixels in the unit.
 5. Anelectronic equipment having a display device, the display device havingpixels arranged in a matrix form, each of the pixels comprising: anelectro-optical element; a drive transistor adapted to drive theelectro-optical element; a holding capacitance adapted to hold an inputsignal voltage supplied to the drive transistor; and a light-shieldingfilm disposed adjacent to a light extraction area, the light extractionarea being smaller than a light emission area of the electro-opticalelement, wherein a capacitance value of the electro-optical elementformed by the light emission area of the electro-optical element is setlarger than a capacitance value of the holding capacitance, a unitincludes at least two pixels that have different light emission areasassociated with different emission colors, and the capacitance value ofthe electro-optical element is the same for the pixels in the unit. 6.The display device of claim 1, wherein the holding capacitance isconnected to a connection node to which the electro-optical element andthe drive transistor are connected.
 7. The display device of claim 6,wherein the holding capacitance is connected between a gate of the drivetransistor and a source of the drive transistor, and the connection nodeis disposed on the source of the drive transistor.
 8. Theelectro-optical element driving method of claim 3 comprising: drivingthe electro-optical elements and bringing drive currents of theelectro-optical elements to approximately the same level by varying thelight emission area between the electro-optical elements.
 9. The displaydevice according to claim 1, wherein each of the pixels in the unit hasa different material and a different film thickness from other pixels inthe unit.
 10. The display device according to claim 2, wherein each ofthe pixels in the unit has a different material and a different filmthickness from other pixels in the unit.
 11. The electro-optical elementdriving method according to claim 3, further comprising providing eachof the pixels in the unit to have a different material and a differentfilm thickness from other pixels in the unit.
 12. The electro-opticalelement driving method according to claim 4, further comprisingproviding each of the pixels in the unit to have a different materialand a different film thickness from other pixels in the unit.
 13. Theelectronic equipment according to claim 5, wherein each of the pixels inthe unit has a different material and a different film thickness fromother pixels in the unit.